"UNDO_TABLESPACE specifies the undo tablespace to be used when an
instance starts up. If this parameter is specified when the instance
is in manual undo management mode, then an error will occur and
startup will fail.

If the UNDO_TABLESPACE parameter is omitted, the first available undo
tablespace in the database is chosen. If no undo tablespace is
available, the instance will start without an undo tablespace. In such
cases, user transactions will be executed using the SYSTEM rollback
segment. You should avoid running in this mode under normal
circumstances.

You can replace an undo tablespace with another undo tablespace while
the instance is running."